Embodiments of the present invention relate generally to electronic sensing devices and particularly to eddy current probes.
Non-Destructive Examination (hereafter “NDE”) of tubing in heat exchangers is essential to maintain the function of the unit, and to maintain the safety of plant workers and of the public. In particular, NDE of steam generators in nuclear plants is essential, as it allows for proactive repair of damaged tubes prior to costly unscheduled outages and/or significant safety-related events. Electromagnetic sensing devices, known as eddy current probes, are primarily used for such NDE. Generally, the probe resides at one end of a cable with the other end presenting an electrical connection to a remote data acquisition unit (RDAU) for collecting and analyzing the signal data. The probe is inserted into heat exchanger tubing and the signals produced thereby are transmitted along the cable and to the remote data acquisition unit. Signals taken from the probe provide indication of tube conditions including a variety of defects and anomalies of interest.
Eddy current probes for tubing can assume a number of different forms. Historically, there have been three general classifications of eddy current probes for heat exchanger and other tubing inspections: bobbin, array, and rotating.
Bobbin probes include a differential pair of annular (ring shaped) coils pulled through a given length of tubing. The coil turns are coaxial with the axis of the tubing. Bobbin probes provide the simplest exam and operate at relatively high speeds. Data acquisition speed is rapid with a simple single channel analysis. This type of probe is most common, as it is simple to build and is least expensive. Due to their simplicity, and due to their potential for testing at high speeds, bobbin probes are used for the majority of heat-transfer areas in a steam generator. There are, however, distinct limits to the size and location of flaws that may be detected with a bobbin probe. Bobbin probes are not adequate for detecting certain types of flaws, or for providing accurate depth and/or length information about a particular flaw. For example, the circumferential windings of a bobbin probe prevent reliable detection of circumferential defects, regardless of their location. For detection of circumferential flaws or smaller axial flaws in areas known to produce unwanted signal noise is required, bobbin probes do not provide acceptable performance. Consequently, certain areas of a steam generator require the use of the more sophisticated rotating or array probes.
Rotating probes involve the use of a motor, usually separate from the probe head, to mechanically spin a smaller, more focused coil within a length of tubing. The whole assembly is withdrawn from the tube at a slow rate of speed, e.g., 0.4 to 0.6 inches per second. As the probe spins it moves along a helical scan path within the tube. When properly used with a trigger pulse channel, rotating probe data may be presented in an isometric view, known as a “C-scan” for enhanced flaw detection and characterization. Although the merits of rotating probe inspection are widely known, the speed of data acquisition and analysis are relatively slow. Acquisition speed using rotating probes can be as much as 100 times slower than bobbin exams. The volume of data required is also much higher. For example, in a ⅞″ diameter tubing a rotating probe travels 876 helical inches to inspect 12 inches of tubing and creates a data file 73 times larger than a bobbin probe data file.
Thus, while significantly slower than bobbin examinations, rotating probe examinations offer enhanced detection, detailed sizing, and better characterization. This is particularly true in the case of circumferential cracks, to which bobbin probes are insensitive. The insensitivity of bobbin probes to circumferential cracks can be attributed to the fact that the coils are wound in the same plane as the defect, resulting in little or no interruption of eddy currents in the tube; a condition required for detection. Bobbin probes are primarily sensitive to axial (longitudinal) cracks, and volumetric defects like pits, while rotating pancake or rotating orthogonal coil arrangements allow for detection and characterization of circumferential cracks.
The presence of circumferential cracks has long been thought to be limited to the “Top-of-Tube Sheet” (TTS) area of steam generators. Consequently, rotating probe examinations have been primarily used only in this small region. One exception includes dented tubes that must be inspected with rotating probes due to bobbin signal interference, or noise, caused by dents. Circumferential cracks have been found, however, in certain nuclear plants. This has created a requirement for all nuclear plants at risk to inspect all U-bends for circumferential cracks. U-bend inspection, however, encompasses a relatively large surface area as compared to the relatively smaller surface area inspected in a TTS inspection. For this reason, detecting circumferential cracks, especially at U-bends, calls for a method of detection faster than that of rotating probes.
Array probes provide a partial solution to the acquisition speed problems posed by rotating probes. By positioning a number of coils in an array around the circumference of a probe, array probes offer 360-degree coverage without the need to rotate a single coil. Array probes may therefore be withdrawn from the tube at a higher rate of speed than rotating probes (typically 10-12 inches per second).
Several different multi-coil array probes have been proposed and used. Generally, these probes offer a pull-through method of inspection with as many as 32 coils designed to duplicate RPC responses at much faster speeds. These array probes can provide adequate detection of circumferential cracks. However, array probes produce a high volume of data, e.g., 32 coil elements operating at 4 frequencies produce 128 data channels. As a result, data analysis becomes time-consuming. Further, array probes are highly sensitive and can reveal flaw indications falling below the interest level of the utility. Nevertheless, all flaw indications must be analyzed and suitably rectified in accordance with applicable regulations and guidelines. This exhaustive and often unproductive work is an unwelcome feature of the array probe inspection method.
A need thereby exists for an eddy current probe capable of detecting a full range of defect types at high inspection speeds without a corresponding high volume of data. Embodiments of the present invention provide such high-speed inspection with reasonable volumes of data.